The present invention relates to a photovoltaic system and more particularly to a photovoltaic system with a plurality of electrically connected photovoltaic modules.
Photovoltaic systems are generally known. These systems typically include a number of photovoltaic modules connected in series to form a so-called string. Each photovoltaic module in turn includes about 100 photovoltaic cells, which are also electrically connected in series. A single photovoltaic cell generates a voltage of about 0.5 V when illuminated by sunlight. As a result, each string has under load a voltage across the string of about 500 V depending on the specific application of the system. This voltage is also referred to as string voltage. In the following example, a string voltage of about 500V under load and of about 800 V under open-circuit conditions (no load) is assumed. Several strings, e.g. 10 strings, may be connected in parallel, with the generated energy then transmitted via a bus for further use.
The generated electrical energy is provided in form of a DC voltage which is converted into an AC voltage by an inverter. Typical conventional exemplary circuit diagrams are shown in FIGS. 1 and 2, where identical components are designated with identical reference symbols
As shown in FIG. 1, the photovoltaic system includes a plurality of photovoltaic cells 3 connected in series and forming, in the illustrated example, two strings 5 which are connected in parallel. The photovoltaic generator 6 formed in this manner has a first string terminal 7 at a negative potential and a second string terminal 9 at a positive potential. The first string terminal is the negative terminal of the photovoltaic generator at a first (lower) potential P1, and the second string terminal is the positive terminal of the photovoltaic generator at a second (higher) potential P2. An inverter 11 is connected to the string terminals 7 and 9. The voltage between the two string ends 7, 9 under load is, as mentioned above, about 500 V.
As illustrated in FIG. 1, the photovoltaic system is operated at a fixed potential reference potential, i.e., the negative potential P1 is connected to ground 13, and the positive potential P2 is, commensurate with the number of serially connected photovoltaic cells of the two strings 5, about 500 V under load.
The basic disadvantage of the circuit of FIG. 1 is its high affinity to “attract” lightning due to the connection of the negative potential to ground. Accordingly, wide-ranging precautions must be taken to prevent lightning strikes which could destroy the inverter, causing a loss of several 100.000  in larger systems. Alternatively, complex overvoltage protection devices would have to be installed, raising the overall price tag of the system 1. The components carrying high voltages must also be protected against accidental contact. There is a danger of an electric shock for a person standing on the ground who touches lines or conductor parts at, for example, the maximum line voltage U0. All bare components installed in the system must therefore be grounded.
It has been observed that a very small, but measurable current can flow from the individual modules 3 to ground 13, but that the modules 3 are not damaged even after prolonged operation under normal conditions.
The second circuit diagram shown in FIG. 2 includes, to simplify the drawing, in the photovoltaic generator 6 only a single string 5 constructed of serially connected modules 3. This photovoltaic system 1 reduces the risk for lightning strikes and eliminates the danger of an electric shock for bystanders, but has another disadvantage. The magnitude of the voltage at each of the two string terminals 7, 9 is about the same with reference to ground 13 in the illustrated so-called potential-free operation of the photovoltaic system 1. The positive potential P2 to ground 13 under open circuit conditions (U0=800 V) is in this example about +400 V, while the negative potential P1 to ground 13 is also about −400 V. These voltages to ground are caused, in spite of the potential-free operation, by a non-negligible relatively small conductance (=the inverse of the ohmic resistance) of the relative the long connecting lines between the modules 3 (wiring of the system 1) and the cables to the inverter 11. The small conductance is symbolized in the schematic circuit diagram by a resistor 14, which is connected approximately between the center of the series connection 5 of the modules 3 and ground 13. Parasitic discharges to ground 13 then become finite and the potential with reference to ground 13 is distributed as mentioned above, i.e., +400 V and −400 V, which represents the most advantageous energy distribution for the overall system 1.
It has been observed that a discharge from anodes, i.e., a discharge from the part of the photovoltaic system 1 with a positive potential to ground, does not damage the affected photovoltaic modules 3. Conversely, a discharge from cathodes causes damage to the photovoltaic modules over an extended period of time, damaging (eroding) the edge region of the TCO layer of the photovoltaic modules 3 and causing a premature permanent power drop. The TCO layer is typically referred to as the semiconductor layer in a module 3 which is disposed between two glass panes. Several exemplary discharges are depicted in FIG. 2 by the arrows 15a, 15b, 17a, 17b. 
It should be mentioned that electrons flow from the top modules 3a at a positive potential to the bottom modules 3b at a negative potential, as shown by arrows 15a. This is referred to as “electron absorption.” The arrows 15a extending from module to module indicate a “cathode discharge.” A small “anode discharge” may also occur as indicated by the module to module arrows 15b pointing in the opposite direction. As already mentioned, “electron absorption” can damage the modules 3b during their service life.
Other “cathode discharges” are symbolized by arrows 17a. In this case, the voltage division by resistor 14 causes electrons to flow from the bottom modules 3b to ground 13. A small “anode discharge” is also present, as indicated by arrows 17b. These “cathode discharges” (arrows 17a) should also be prevented if possible.
In case of an electrical fault, conventional equipment with potentially exposed electrical high-voltage components puts service personnel at risk. A check can only be performed with a voltage tester and by systematically contacting all conductors. This may take several weeks in large high-power photovoltaic systems and is therefore not practical.
In addition, it is also not possible to detect when one or more parallel-connected strings 5 of photovoltaic modules 3 are disconnected, either because these modules 3 became defective or due to theft.
Accordingly, there is a need to keep cathode discharges on modules as small as possible or to eliminate them entirely, and to provide the system against lightning strikes. There is further a need to protect personnel from accidental electric shock and to detect theft.